01 - In a large bowl, whisk together the flour, sugar, yeast, and salt. Combine the warm milk and egg in a separate container, then pour into the dry mixture. Stir until a shaggy dough begins to form. Add the softened butter and knead by hand or with a stand mixer fitted with a dough hook for 8 to 10 minutes, until the dough is smooth, elastic, and slightly tacky. Cover the bowl with a damp towel or plastic wrap and let rise in a warm spot for 1 hour, or until doubled in size.
02 - Place ¾ cup plus 1 ½ tablespoons of room-temperature butter on a sheet of parchment paper. Using a rolling pin, shape it into a rectangle approximately 8x10 inches. Transfer to the refrigerator and chill until firm yet still pliable, about 15 to 20 minutes.
03 - On a lightly floured surface, roll the risen dough into a large rectangle approximately 16x12 inches. Lay the chilled butter sheet over one half of the dough, then fold the other half over to enclose the butter. Roll out gently to seal, then fold the dough into thirds like a letter. Wrap in plastic and chill for 20 minutes. Repeat the rolling and folding process two more times, chilling for 20 minutes between each turn to keep the butter cold.
04 - Roll the laminated dough into a rectangle approximately 16x12 inches. Trim the edges for clean, even lines. Cut the dough vertically into 12 equal strips, each about 1 ⅓ inches wide. If using a filling, spread a thin, even layer of chocolate spread or jam down the length of each strip.
05 - Roll each strip tightly from one end to the other, forming a compact spiral. Using a sharp knife, slice each spiral in half lengthwise to expose the layered interior. Twist each half into a loose knot and place cut-side up into a generously greased 12-cavity muffin tin.
06 - Cover the muffin tin loosely with oiled plastic wrap and let the shaped cruffins rise in a warm, draft-free area for 45 to 60 minutes, until noticeably puffy and nearly doubled.
07 - Preheat the oven to 390°F (200°C). Bake the cruffins on the center rack for 20 to 25 minutes, rotating the tin halfway through, until the tops are a deep golden brown and the layers are visibly flaky.
08 - Remove from the oven and immediately brush the warm cruffins with melted butter, then roll in cinnamon sugar if desired. Allow to cool in the tin for 5 minutes before transferring to a wire rack. Serve as is, or pipe lemon curd, Nutella, or pastry cream into the center for an extra indulgent treat.